1. Field of the Invention
This invention relates to a group III-V nitride-based semiconductor substrate and a method of making the same.
2. Description of the Related Art
GaN-based compound semiconductors such as gallium nitride (GaN), indium gallium nitride (InGaN) and aluminum gallium nitride (AlGaN) attract attention for a material of blue light emitting diode (LED) or laser diode (LD). Further, since the GaN-based compound semiconductors have a good heat resistance and environment resistance, they have begun to be applied to other electronic devices.
At present, a substrate used widely to grow GaN is sapphire. In general, a method is used in which GaN is epitaxially grown on a sapphire single crystal substrate by MOVPE (metalorganic vapor phase epitaxy) etc.
However, since the sapphire substrate mismatches in lattice constant with the GaN, a GaN single crystal cannot be grown directly on the sapphire substrate. Therefore, a method is disclosed in which a buffer layer (=low-temperature growth buffer layer) of AlN or GaN is grown on the sapphire substrate at a low temperature to buffer strain in lattice and then GaN is grown thereon (e.g., Japanese patent Nos. 3026087 and 2751963 and Japanese patent publication No. 8-8217).
By using the low-temperature growth buffer layer, the epitaxial growth of GaN single crystal can be realized. However, the above method still has a problem that the grown GaN has a number of defects since the lattice mismatch between the substrate and the GaN is not eliminated. It is presumed that the defect brings some failure to a manufacture of GaN-based LD.
Under the circumstances, it is desired to develop a GaN self-standing substrate. Since it is difficult to grow a large ingot of GaN from melt unlike Si or GaAs, various methods such as the ultrahigh temperature and pressure method, flux method and HVPE (hydride vapor phase epitaxy) have been tried to make the GaN self-standing substrate.
A typical method for making a nitride semiconductor self-standing substrate is conducted such that a GaN thick film is grown on a hetero-substrate such as sapphire by HVPE and then the hetero-substrate is removed to obtain a GaN self-standing substrate (e.g., JP-A-2003-178984, herein referred to as VAS method and the entire contents of JP-A-2003-178984 are incorporated herein by reference). In this method, a void-containing layer functions as a strain buffering layer so as to buffer a strain caused by a difference in lattice constant or thermal expansion coefficient between the underlying substrate and the group III nitride semiconductor layer grown thereon. By the method, a substrate of group III nitride semiconductor can be obtained which offers a reduced defect density and a good crystalline quality without warping. Further, the self-standing substrate thus obtained can be easily separated. Based on the method, GaN substrates with a reduced dislocation have begun to be commercially available.
However, a large practical GaN single crystal with a high crystalline quality has never been developed even in the above methods.
In the ultrahigh temperature and pressure method, which needs tens of thousands of atmospheres and thousands of degrees, it is difficult to grow a large crystal. Therefore, it only can provide a GaN crystal with a diameter of several millimeters and a thickness of several tens of millimeters.
In the flux method, although it only needs hundreds of atmospheres and about a thousand degrees, it only can provide a GaN crystal with a diameter of several millimeters and a thickness of several tens of micrometers. In addition, there are problems that removal of nitrogen occurs and Na or Ca flux is diffused into the crystal. Furthermore, since it is difficult to control the generation of crystal nuclei at initial growth, polycrystal may be contained.
In the HVPE method, a crystal with a diameter of about 5.08 cm(=2 inches) has been developed. However, in view of economical aspect of device fabrication, a lager wafer of GaN single crystal is desired with a diameter of 7.62 cm(=3 inches) or more. In fabricating such a large wafer, there is a problem that in-plane properties thereof become significantly non-uniform and thereby the large area becomes meaningless. For example, the non-uniformity of dislocation density may cause dispersion in reliability of each device. The non-uniformity of electrical resistivity (carrier concentration) may cause dispersion in operating voltage. The non-uniformity of crystal orientation may cause dispersion in emission wavelength since the InGaN composition of an active layer may be non-uniform. The non-uniformity of thickness, especially unevenness on the back surface face of the substrate causes non-uniformity of temperature distribution during growth of device epitaxial layer. This affects the InGaN composition of the active layer to cause non-uniformity in the emission wavelength.